Woven wire cloth screen

ABSTRACT

A woven wire cloth screen is described in which at least one fine mesh cloth overlies a coarser mesh backing cloth and an additional coarser mesh cloth is secured over the fine mesh cloth, whose mesh and wire size at least are selected so as in general to prevent relatively large abrasive particles from making contact with the fine mesh cloth. Two fine mesh cloths may be sandwiched between the coarser cloths. In one screen the backing cloth is 30#×01.280 mm diameter S/S wire, the upper fine mesh is 180#×0.030 mm diameter S/S wire, the lower fine mesh is 160#×0.036 mm diameter S/S wire, and the additional top cloth is 30#×60#×0.160 mm diameter S/S wire. A method of making such a screen is described in which the cloths are in turn laid over a rigid frame to which they are to be bonded and the tension in the wire mesh cloths is achieved using pneumatically powered rams acting on clamps which grip the edges of the cloths. All the rams are of the same size and different tensions are imparted to the cloths by supplying air at differing pressures to the rams acting on the different cloths. In particular a method is described for making a woven wire cloth screen which will withstand highly abrasive large particles such as obtained when drilling through Utsira sand, in which a support frame is located in a jig, a coarse backing cloth is stretched over the frame -and secured between clamping jaws along all four edges and then tensioned by moving all the clamps relatively outwardly by pneumatic rams. A fine mesh wirecloth is then stretched over the first cloth and similarly tensioned by means of a second set of clamps and rams. An additional coarse mesh cloth is then similarly stretched over the fine mesh cloth and tensioned by means of a third set of clamps and rams. Thereafter the cloths are secured to the support frame under pressure. After releasing the pressure, opening the clamping jaws, and trimming surplus wirecloth back to the frame, the finished screen can be removed from the jig. A second fine mesh cloth can be laid over the first fine mesh cloth before the top coarse cloth is applied, and the two fine mesh cloths can be clamped using the same set of jaws and rams.

FIELD OF THE INVENTION

This invention concerns woven wire cloth screens for filtering liquidsfrom a mixture of solids and liquids, particularly but not exclusivelydrilling muds. Typically such screens are located in a vibratory shakerto assist in separating the constituents of the mud and to cause thesolids left on the wire cloth of the screen to migrate to one endthereof for discharge and collection.

BACKGROUND

Certain strata produce relatively large highly abrasive particles whendrilled, and screens have been found to fail prematurely in use, whendrilling through such material. Thus instead of a screen lasting for upto several days before abrasion ruptures the wire cloth, failure hasoccurred within a few hours.

It is an object of the present invention to provide an improved designof woven wire cloth screen, which overcomes this problem.

SUMMARY OF THE INVENTION

According to the present invention, in a woven wire cloth screen inwhich at least one fine mesh cloth overlies a coarser mesh backingcloth, an additional coarser mesh cloth is secured over the fine meshcloth, whose mesh and wire size at least are selected so as in generalto prevent relatively large abrasive particles from making contact withthe fine mesh cloth.

In general the tension in wires making up one cloth is arranged to bedifferent from the tension in the wires making up the other cloth andwhere there are three or more cloths, different tensions may exist ineach of the cloths. However it is to be understood that the invention isnot limited to arrangements in which each cloth has a different tensionfrom the other(s) and the cloths (or some of them) may have the sametension if appropriate.

The tension of the said additional cloth can affect its performance, andthe tension in the mesh of this additional cloth is selected so that itwill perform its protective function and not adversely affect thevibration of the fine mesh cloth (or cloths) therebelow.

In general the tension in the additional coarse mesh cloth is greaterthan that in the fine mesh cloth(s), typically approximately twice thetension in the latter.

Preferably two fine mesh cloths overlie the backing cloth so that onefine mesh cloth is sandwiched between the upper fine mesh cloth and thebacking cloth, and the mesh size of the cloth which is so sandwiched isvery close to the mesh size of the said upper cloth, and in use assistsin de-blinding the upper cloth due to relative vibrational movementbetween it and the lower fine mesh cloth, and the additional coarsermesh cloth overlies the upper fine mesh cloth.

The mesh size of the additional cloth must be selected with care sinceif the mesh openings are too large, correspondingly large sharpparticles can reach and damage the upper cloth therebelow. Likewise ifthe openings in the additional cloth are too large the additional clothmay not allow steady migration of separated solids over the screen inuse, resulting inter alia in local overloading and failure.

The additional cloth may to advantage have rectangular mesh openingssuch that for example the openings in the weave are twice as long in onedirection as in the other. In general the larger dimensions of theopenings in the weave should run parallel to the direction in whichseparated solids migrate over the surface of the screen in use.

Typically the cloths are stretched over and bonded to a rigid frame andin general such frames are rectangular (although other shaped frames canbe used). A screen having a rectangular frame will be referred to as arectangular screen and in general rectangular screens are normallyoperated so that the longer dimension of the screen is parallel to thedirection in which separated solids migrate over the screen. Thereforeas a general rule, where the screen includes a generally rectangularframe to which the cloths are secured, the said additional cloth isfitted to the screen frame so that the larger dimensions of the openingsin the weave are parallel to the longer edges of the frame.

Where a screen frame is for example square, and the cloths haverectangular openings, the cloths are preferably fitted to the frame sothat the larger dimensions of the mesh openings are parallel to one edgeof the frame and the latter is mounted in a shaker so that the directionof solids migration over the screen in use is parallel to the said oneedge of the frame.

Examples of Improved Screens

In a preferred screen having a single fine mesh cloth the three clothsmay be as follows:— Backing cloth: 30# × 0.280 mm diameter S/S wire Finemesh cloth: 180# × 0.030 mm diameter S/S wire Additional top cloth: 30#× 60# × 0.160 mm diameter S/S wire.

In another preferred screen having two fine mesh cloths, the four clothsmay be as follows:— Backing cloth: 30# × 01.280 mm diameter S/S wireUpper fine mesh: 180# × 0.030 mm diameter S/S wire Lower fine mesh: 160#× 0.036 mm diameter S/S wire Additional top cloth: 30# × 60# × 0.160 mmdiameter S/S wire

Tensions in the wire mesh cloths is normally achieved usingpneumatically powered rams acting on clamps which grip the edges of thecloths. If (as is usual) all the rams are of the same size, the tensionsimparted to the cloths will be proportional to the air pressure appliedto the rams acting on the different cloths.

In the case of a three-cloth screen the pressures applied to the threesets of rams may be in the following ratios:— Backing mesh Fine meshAdditional top mesh 2.0 1.0 1.9

In the case of a four cloth screen the pressures applied to thedifferent sets of rams may be in the following ratios: Backing meshLower fine mesh Upper fine mesh Additional top mesh 2.0 1.0 1.0 1.9

Where the two fine mesh cloths are to have the same tension (as in thislast example) they can be tensioned together using the same set ofclamps and rams.

If however the tension in one fine mesh cloth is to be different fromthat in the other fine mesh cloth, then a further set of rams and clampswill be needed to allow each cloth to be tensioned independently of theother.

A screen constructed in accordance with the invention has proved moredurable than screens not having the additional cloth when siftingdrilling mud recovered from a sea-bed drilling operation as the drill isprogressing through sub-sea strata composed of Utsira sand.

The invention also lies in a method of making a woven wire cloth screenwhich will withstand highly abrasive large particles such as obtainedwhen drilling through Utsira sand, in which a support frame is locatedin a jig, a coarse backing cloth is stretched over the frame and securedbetween clamping jaws along all four edges and then tensioned by movingall the clamps relatively outwardly by pneumatic rams, similarlystretching a fine mesh wirecloth over the first cloth and similarlytensioning it by means of a second set of clamps and rams, similarlystretching an additional coarse mesh cloth over the fine mesh cloth andtensioning it by means of a third set of clamps and rams, thereaftersecuring the cloths to the support frame under pressure, releasing thepressure, opening the clamping jaws, trimming surplus wirecloth back tothe frame and removing the finished screen from the jig.

If two fine mesh cloths are required the second fine mesh cloth may belaid over the first fine mesh cloth before the top coarse cloth isapplied. Both fine mesh cloths can be clamped with the same set of jawsand rams, or a further set of jaws and rams may be provided fortensioning the second fine mesh cloth. Where only one set of jaws isprovided for the two fine mesh cloths the two cloths will have the sametension, but where a separate set of jaws and rams are provided thetension in the second fine mesh cloth may be different from that in thefirst fine mesh cloth.

If slippage is noted where two fine mesh cloths are clamped in the sameset of jaws, the method may include the step of inserting a strip ofcoarser mesh cloth between the two layers locally between the clampingjaws, to assist in gripping the two fine mesh layers between the jaws.

The invention will now be described by way of example with reference tothe accompanying drawing in which the single FIGURE, FIG. 1, illustratesto an enlarged scale in plan view from above, a small part of a finemesh cloth 10 overlaid by a coarse mesh cloth 12. The mesh size of 12 issuch as to prevent, in general, relatively large abrasive particles frommaking contact with the fine mesh cloth 10. In the drawing, a backingcloth (which would be below the fine mesh cloth) has been omitted, forclarity.

All of the cloths are secured in position in manner known per se to arelatively rigid frame (not shown) by bonding to a thermoplasticmaterial from which the screen is constructed or which covers a metal orother inner core, so as to form a screen for use in a sieving machinesuch as a shale shaker used to separate solids from drilling mudretrieved from down-hole during oil or gas well drilling operations.

An example of such a screen and frame and its manner of manufacture isillustrated and described in GB Patent Specification No. 2,322,590 andGB Patent Application No. 0226338.2.

1. A woven wire cloth screen in which at least one fine mesh clothoverlies a coarser mesh backing cloth and an additional coarser meshcloth is secured over the fine mesh cloth, whose mesh and wire size atleast are selected so as in general to prevent relatively large abrasiveparticles from making contact with the fine mesh cloth.
 2. A screen asclaimed in claim 1 wherein the tension in wires making up one cloth isdifferent from the tension in the wires making up at least one of theother cloths.
 3. A screen as claimed in claim 2 wherein a differenttension exists in the wires making up each of the cloths.
 4. A screen asclaimed in claim 1 wherein the tension in the wires of each of thecloths is the same.
 5. A screen as claimed in claim 1 wherein thetension in the additional coarse mesh cloth is greater than that in thefine mesh cloth(s).
 6. A screen as claimed in claim 5 wherein thetension in the additional coarse mesh cloth is approximately twice thetension in the fine mesh cloths.
 7. A screen as claimed in any of claims1 to 6 claim 1 wherein two fine mesh cloths overlie the backing cloth sothat one fine mesh cloth is sandwiched between the upper fine mesh clothand the backing cloth, and the mesh size of the cloth which is sosandwiched is very close to the mesh size of the said upper fine meshcloth, and in use assists in de-blinding the upper cloth due to relativevibrational movement between it and the lower fine mesh cloth, and theadditional coarser mesh cloth overlies the upper fine mesh cloth.
 8. Ascreen as claimed in claim 1 wherein the additional cloth hasrectangular mesh openings.
 9. A screen as claimed in claim 8 wherein therectangular openings in the weave are twice as long in one direction asin the other.
 10. A screen as claimed in claim 8 wherein the largerdimensions of the openings in the weave run parallel to the direction inwhich separated solids migrate over the surface of the screen in use.11. A screen as claimed in claim 1 wherein the cloths are stretched overand bonded to a rigid frame.
 12. A screen as claimed in claim 11 whereinthe rigid frame is rectangular.
 13. A screen as claimed in claim 12which in use is arranged so that the longer dimension of the screen isparallel to the direction in which separated solids migrate over thescreen.
 14. A screen as claimed in claim 8 wherein the additional clothis fitted to the frame so that the larger dimensions of the openings inthe weave are parallel to the longer edges of the frame.
 15. A screen asclaimed in claim 1 having a single fine mesh cloth sandwiched betweenbacking cloth and overlying additional coarse mesh cloth in which thebacking cloth is 30#×0.280 mm diameter S/S wire, the fine mesh cloth is180#×0.030 mm diameter S/S wire and the additional top cloth is30#×60#×0.160 mm diameter S/S wire.
 16. A screen as claimed in claim 7wherein the backing cloth is 30#×01.280 mm diameter S/S wire, the upperfine mesh is 180#×0.030 mm diameter S/S wire, the lower fine mesh is160#×0.036 mm diameter S/S wire, and the additional top cloth is30#×60#×0.160 mm diameter S/S wire.
 17. A method of making a screen asclaimed in claim 1 wherein the cloths are in turn laid over a rigidframe to which they are to be bonded and the tension in the wire meshcloths is achieved using pneumatically powered rams acting on clampswhich grip the edges of the cloths, wherein all the rams are of the samesize and different tensions are imparted to the cloths by supplying airat differing pressures to the rams acting on the different cloths.
 18. Amethod as claimed in claim 17 when employed to make a screen as claimedin claim 1 in which the pressures applied to the three sets of rams arein the following ratios, backing mesh rams 2.0, fine mesh rams 1.0, andadditional top mesh rams 1.9.
 19. A method as claimed in claim 17 whenemployed to make a screen as claimed in claim 7 in which the pressuresapplied to the rams are in the following ratios, backing mesh rams 2.0,lower fine mesh rams 1.0, upper fine mesh rams 1.0, and additional topmesh rams 1.9.
 20. A method as claimed in claim 19 wherein the two finemesh cloths are clamped and tensioned together, using one set of clampsand rams.
 21. A method of making a woven wire cloth screen which willwithstand highly abrasive large particles such as obtained when drillingthrough Utsira sand, in which a support frame is located in a jig, acoarse backing cloth is stretched over the frame and secured betweenclamping jaws along all four edges and then tensioned by moving all theclamps relatively outwardly by pneumatic rams, similarly stretching afine mesh wirecloth over the first cloth and similarly tensioning it bymeans of a second set of clamps and rams, similarly stretching anadditional coarse mesh cloth over the fine mesh cloth and tensioning itby means of a third set of clamps and rams, thereafter securing thecloths to the support frame under pressure, releasing the pressure,opening the clamping jaws, trimming surplus wirecloth back to the frameand removing the finished screen from the jig.
 22. A method as claimedin claim 21 wherein a second fine mesh cloth is laid over the first finemesh cloth before the top coarse cloth is applied.
 23. A method asclaimed in claim 22 wherein the two fine mesh cloths are clamped usingthe same set of jaws and rams.
 24. A screen as claimed in claim 1 whenfitted to and used to sift drilling mud recovered from a sea-beddrilling operation progressing through sub-sea strata composed of Utsirasand.